docs: def heightmap_rain_erosion(
      hm: np.ndarray,
      nbDrops: int,
      erosionCoef: float,
      sedimentationCoef: float,
      rnd: Optional[tcod.random.Random] = None,
  ) -> None:
      """Simulate the effect of rain drops on the terrain, resulting in erosion.
  
      ``nbDrops`` should be at least hm.size.
  
      Args:
          hm (numpy.ndarray): A numpy.ndarray formatted for heightmap functions.
          nbDrops (int): Number of rain drops to simulate.
          erosionCoef (float): Amount of ground eroded on the drop's path.
          sedimentationCoef (float): Amount of ground deposited when the drops
                                     stops to flow.
          rnd (Optional[Random]): A tcod.Random instance, or None.
      """
      lib.TCOD_heightmap_rain_erosion(
          _heightmap_cdata(hm),
          nbDrops,
          erosionCoef,
          sedimentationCoef,
          rnd.random_c if rnd else ffi.NULL,
      )